Printing presses of the prior art have been equipped with densitometers that illuminate the printed paper at the output of the press and measure the light reflected from the paper in order to measure the optical reflection density of ink being printed by the press. An operator can use the ink density reading as a guide for adjusting the rate at which ink is supplied to the printing press, to achieve a desired density of printed ink. One such densitometer is described in U.S. Pat. No. 3,756,725, issued Sept. 4, 1973, by J. Manring, and entitled Measurement and Control of Ink Density, which is incorporated herein by reference.
In other prior art it has been suggested that a flexible light pipe could be used to conduct light from a stationary lamp to a movable densitometer head to read density at various places, or to conduct reflected light received by a movable densitometer head to a stationary photosensitive detector.
Systems of the prior art are often so bulky that it is difficult to mount them on a printing press in a place where a reliable measurement of ink density can be obtained and where it is practical to replace color filters of the densitometer head to measure different colors of ink. Another problem that has been troublesome in densitometer heads of the prior art is that the lenses of the optical system become dirty very rapidly when the press is running, because of settling of airborne dry ink dust and of powder that is applied to the wet ink after printing to prevent undesired transfer of the ink. This necessitates frequent cleaning of the lenses in order to maintain the efficiency of the densitometer.